Question: You have found the following ages (in years) of 4 snakes. The snakes are randomly selected from the 39 snakes at your local zoo: $ 5,\enspace 5,\enspace 7,\enspace 20$ Based on your sample, what is the average age of the snakes? What is the variance? You may round your answers to the nearest tenth.
Answer: Because we only have data for a small sample of the 39 snakes, we are only able to estimate the population mean and variance by finding the sample mean $({\overline{x}})$ and sample variance $({s^2})$ To find the sample mean , add up the values of all $4$ samples and divide by $4$ $ {\overline{x}} = \dfrac{\sum\limits_{i=1}^{{n}} x_i}{{n}} = \dfrac{\sum\limits_{i=1}^{{4}} x_i}{{4$ To compensate for this underestimation, rather than simply averaging the squared deviations from the mean , we total them and divide by $n - 1$ $ {s^2} = \dfrac{\sum\limits_{i=1}^{{n}} (x_i - {\overline{x}})^2}{{n - 1}} $ $ {s^2} = \dfrac{{18.49} + {18.49} + {5.29} + {114.49}} {{4 - 1}} $ $ {s^2} = \dfrac{{156.76}}{{3}} = {52.25\text{ years}^2} $ We can estimate that the average snake at the zoo is 9.3 years old. There is a variance of 52.25 years $^2$.